The turnover of cells in tissues during normal homeostasis appears to play a fundamental role in immune tolerance. As the cells die by apoptosis, antigens associated with the apoptotic bodies are then taken up and presented to the immune system in a tolerogenic fashion. While this process has been established in several studies, little is known about precisely what the apoptotic body does to the antigen-presenting cell (APC) to promote tolerance. Here we will explore the novel hypothesis that apoptotic caspase proteases in the apoptotic body promote changes that affect the APC from within the endosome. This will be studied in the context of a model in which transgenic viral antigens on pancreatic b-islet cells are targeted by an anti-viral immune response to produce diabetes, and administration of apoptotic cells bearing the viral antigen prevents this autoimmune response. This model system has the advantage that the autoimmune response is rapidly induced and can be monitored at the level of the anti-viral cytotoxic T-lymphocytes (CTL) response as well as diabetes. In particular, we will: 1. Identify the role of apoptosis in targeting antigen to specific APCs capable of inducing tolerance. This aim will establish the tolerance system and the role of apoptosis in the effect, as well as determining if apoptotic bodies bring the antigen to specialized APCs necessary for the induction of tolerance. This aim will set us up for the more speculative second aim, which is to: 2. Explore the impact on antigen presentation of engulfed apoptotic bodies and caspase-dependent events within the endosomes. Here we will determine the role of caspases in the effects of apoptotic bodies on tolerance induction, and test several hypotheses for how caspase-dependent events in apoptosis may affect the cell that engulfs the dying cell. For example, we will the test the novel idea that caspase-induced mitochondrial production of reactive oxygen species facilitates trans-endosomal movement of antigen to the class I MHC pathway in APC that do not normally show such cross-presentation. This and other ideas on the role of caspases in the effects of apoptotic bodies on tolerance induction will open the way to mimic this tolerogenic effect of apoptotic bodies, thereby facilitating the normal process of tolerance induction as a consequence of tissue turnover.